bump minimum go version to 1.15

this is due to a newer version of golang.org/x/net using
os.ErrDeadlineExceeded, which was added in go1.15.

.github/workflows/tests.yml

@@ -22,7 +22,7 @@ jobs:
           # minimum go version that works. This is not necessarily supported in
           # any way, and will be bumped up without notice as needed.  But it at
           # least lets us know what go version should work.
-          - go-version: 1.13
+          - go-version: 1.15
             platform: ubuntu-latest
 
           # include windows, but only with the latest Go version, since there